1. Field of the Invention
This invention relates to an electron spectroscopy apparatus that can obtain information relating to the chemical state of elements in a specimen by analyzing the energy spectrum of photoelectrons generated when the specimen is irradiated, for example by EUV (extreme ultraviolet rays).
2. Description of the Prior Art
Irradiating a substance with visible light, EUV, X-rays, electrons or the like causes ejection of electrons from the substance. The energy distribution of these ejected electrons contains information relating to the state of the substance. Auger electron spectroscopy which measures Auger electrons generated by electron beam radiation, and photoelectron spectroscopy which measures photoelectrons generated by EUV or X-ray radiation, are important analytical techniques in research relating to the state of substances, especially the surface state. Photoelectron spectroscopy is the more important of these techniques.
Photoelectron spectroscopy will now be explained with reference to FIG. 1. When X-ray 2 having a photon energy E.sub.X impinge on a target specimen 1, electrons 3 in the specimen 1 are excited by receiving the energy from the X-ray 2, whereby the X-ray 2 are thus absorbed. If the photon energy E.sub.X of the X-ray 2 is greater than the binding energy E.sub.B of the electrons 3, electrons 3 are discharged from the specimen 1 as photoelectrons 4 with an energy of E.sub.p =E.sub.X -E.sub.B. The basic principle of photoelectron spectroscopy is that in the case of X-ray the beam can be monochromatic, and as photon energy E.sub.X can be known with a high degree of precision, the binding energy E.sub.B of electrons 3 in the specimen 1 can be established with a high degree of resolution by measuring with high precision the energy E.sub.p of the photoelectrons 4. As photoelectrons 4 outside the specimen 1 are released from the outermost surface layer, they can provide information about the state of the surface. With the advances being made in the development of thin-film devices and surface functions therefore becoming increasingly important, information relating to surfaces is also becoming more and more important.
Compared with Auger electron spectroscopy, the advantageous features of X-ray photoelectron spectroscopy are that as it offers a higher energy resolution it can therefore provide much more detailed information, it does little damage to the specimen, and it can be used to observe insulators. A drawback of X-ray photoelectron spectroscopy has been its low spatial resolution. Generally speaking, specimens are spatially non-uniform and have to be analyzed at the microscopic level. With process technologies dealing with ever finer dimensions, microanalysis is becoming increasingly necessary.
Advances have been made with research and development related to obtaining spatially-resolved photoelectron spectra, and have led to the development of a direct imaging technique in which a photoelectron image is magnified and a scanning technique in which spatial distribution is obtained by scanning a location on the specimen with a tightly focused X-ray beam.
The difficulty of focusing X-rays gave rise to the direct imaging method. In U.S. Pat. No. 4,680,467, for example, X-rays of 1,487 eV are focused into a large diameter of 100 to 600 .mu.m using a crystal spectroscope. Thus, this prior art presents a need for a process for electrostatically or magnetostatically enlarging the image of spatial distribution of photoelectrons produced from a relatively large region irradiated by the X-rays in order to obtain detailed spatial distribution of photoelectrons. Progress has proceeded to the point at which, with an X-ray tube as the X-ray source and using an electrostatic lens, the spatial distribution of photoelectrons has been measured with a resolution of 20 .mu.m (P. Coxon et al., J. Elect. Spectrosc. and Rel. Phenom. 52 821 (1990)), and lines of 25 .mu.m width have been observed with synchrotron radiation as the X-ray source and using a magnetic field to produce enlarged photoelectron images (P. Pianetta et al., J. Elect. Spectrosc. and Rel. Phenom. 52 797 (1990)).
Although the direct imaging method has achieved a spatial resolution to some extent, it is very difficult for such method to achieve a sub-micron resolution which is strongly needed in today's research. On the other hand, EUV optic device technology has recently advanced to the point where EUV having a wavelength of several nanometers (several hundreds of eV in photon energy) can be focused down to a beam just several tens of nanometers in diameter, thereby making it possible to use a method for focusing EUV and scanning the focused EUV with an optic device to obtain photoelectron spectroscopy with a sufficiently high resolution. However, it is not easy to emit intensive EUV from an X-ray tube which has heretofore been used as an X-ray source. With an undulator beam which is now an ultra-highly intensive light source of a synchrotron radiation facility as the EUV source and a zone plate or Schwarzschild optics to concentrate the beam, photoelectron images have been obtained with a spatial resolution of up to 0.3 .mu.m (H. Ade et al., Appl. Phys. Lett. 56 1841 (1990)). Photoelectron microscopes are attracting attention as one of the most promising undulator beam applications, and is an area in which research is expected to make considerable progress.
However, using an undulator beam as the EUV source presents a major obstacle to wider use of the minute analysis photoelectron microscopy. This is because the facility is quite bulky and costly, the beam cannot be easily switched on and off, so that in order to use the facility efficiently it has to be used continuously throughout the day, and it requires special operators. These reasons put upkeep of a synchrotron facility beyond the capacity of a small-scale user group.
Moreover, even with an undulator beam, photoelectron spectroscopy of intermittent phenomena is difficult. One example is photoelectron spectroscopy of clusters. A cluster is a molecular compound having a state that is midway between molecular and solid. Research into the properties of clusters is not only highly interesting from the academic point of view but also has the potential of giving rise to new applications, and it is for this reason that cluster research is attracting attention. Virtually all clusters are intermittently produced in very small quantities, for example by irradiating a solid with a pulsed laser beam. Because clusters are produced intermittently, only quite a small fraction of time of the continuously emitting undulator beam is utilized for the measurement. Also, time-of-flight spectroscopy, which is capable of detecting photoelectrons with a very high efficiency, cannot be employed with the continuous irradiation of an undulator X-ray source.
The principal object of this invention is therefore to provide a compact, high-efficiency electron spectroscopy apparatus that is capable of performing high spatial resolution measurements in a short data acquisition time.
A further object of the invention is to provide an electron spectroscopy apparatus that can effectively perform photoelectron spectroscopy of intermittent phenomena such as in cluster observation.